I think every author has individual strengths and weaknesses. Brandon writes amazing worlds and characters, Tolkein was unique in his level of worldbuilding depth, Rothfuss has incredible prose, etc. I think Rowling's strength was her characters. I think all the people in the book, even the background characters, were well written and interesting. Half Blood Prince is the first book I ever remember where I was actually invested in a romantic relationship.
The problems I have with the book are everything else. Her plot is inconsistent at best, she foreshadows well but pays it off badly, and her worldbuilding isn't just shallow, it legitimately has a bunch of problems that ruin the book for a lot of people (like house elves or all Slytherins being evil)